The present invention relates to a method of and an apparatus for controlling a calling signal or current for use in a private branch exchange (PBX) and/or a key telephone set, the calling signal being transmitted from an extension or intercommunicating circuit to a telephone set.
Heretofore, according to the method of controlling the calling signal of this type, an intercommunication circuit number is related to a phase number of the calling signal with a unique correspondence established therebetween; moreover, a hardware device is employed to supply a synchronization signal representing a first phase of the calling signal.
FIG. 5 is a block diagram schematically showing the circuit constitution of a private branch exchange and/or a key telephone set.
This constitution includes trunk line circuits 11, 12, and 13, extension circuits 2l to 2n, telephone sets 3l to 3n, a call channel link 4, a calling signal generator 5, a synchronization signal generator 51 supplying extension controllers 62 and 63 with a synchronization signal indicating a start point of the first phase according to a relationship with respect to time shown in FIG. 6, a central controller 6, a trunk line controller 61, and the extension controllers 62 and 63. Table 1 shows correspondence relationships between extension circuit numbers and ringing phases stored in a main memory of each of the extension controllers 62 and 63. The contents of this table are accessed to control the interconnection circuits 2l to 2n.
TABLE 1 ______________________________________ First phase Second phase Third phase ______________________________________ Ext. circuit 21 Ext. circuit 22 Ext. circuit 23 Ext. circuit 24 Ext. circuit 25 Ext. circuit 26 Ext. circuit 27 Ext. circuit 28 Ext. circuit 29 (Same as above) Ext. circuit Ext. circuit Ext. circuit (3a-2) (3a-1) 3a ______________________________________
That is, as can be seen from this table, the correspondences between the extension circuits and the first, second, and third phases are established in a cyclic or periodical manner as 21-first, 22-second, 23-third, 24-first, 25-second . . . .
FIG. 6 is a signal timing chart representing relationships with respect to time between calling signals produced from the calling signal generator 5 and the synchronization signal created from the synchronization signal generator 51. As shown in the signal timing chart of FIG. 6, phase numbers are associated with respective calling signals transmitted sequentially at one second intervals in this example.
A description will now be given of the operation achieved by the conventional system example above.
In the constitution of FIG. 5, for example, when a telephone set 3e (not shown) is set to an off-hook state to call a telephone set 32 through a dialing operation, the central control unit 6 instructs the extension controller 62 to send a calling tone signal to the extension circuit 22. In response thereto, the extension control unit 62 accesses the main memory thereof to acquire the contents of the correspondence table (Table 1), which denotes that the calling signal to be transmitted to the extension circuit 22 is associated with the second sounding phase. In consequence, as shown in FIG. 6, after one second has elapsed from a reception of a synchronization signal from the synchronization signal generator 51, the extension circuit 22 is activated to send to the telephone set 32 the ringing signal for one second and then to interrupt the signal transmission for two seconds. Thereafter, on receiving the synchronization signal again, the operation above is repeatedly accomplished, namely, after one second has elapsed from the reception of the synchronization signal from the generator 51, the extension circuit 22 is controlled to transmit the ringing signal to the telephone set 32 for one second and then to interrupt the signal transmission for two seconds. As a result, the telephone set 32 is caused to sound through the repetitious operations above. That is, the telephone set 32 sounds in accordance with a ringing pattern of 1-second ON (sound) and 2-second OFF (silent).
As above, also in the conventional method of controlling the ringing signal, when the ringing operations are conducted in a random manner, the calling signals thus transmitted are most efficiently distributed to the ringing phases 1 to 3.
Moreover, in the prior art example, the calling signal has a period associated with that of the synchronization signal created from the synchronization signal generator 51, which limits the ringing, pattern to, for example, 1-second ON and 2-second OFF.
However, according to the conventional method of controlling the calling signal, there may possibly occur a situation where the load is concentrated on one of the ringing phases in some cases. In order to overcome this difficulty, the calling signal generator 5 is required to possess an output capacity coping with the centralized load.
For example, assuming here that a calling signal power of one watt (W) is necessary for each extension circuit, when only the extension circuits 21, 4, 27, . . . , (3a-2) undergo a ringing operation, in other words, one third of the total number of the extension circuits are activated for the sounding operation, the calling signal generator 5 is required to supply a power of n/3 watt (n stands for the total number of the telephone sets connected to the calling signal generator 5) for one third of the period. That is, when all telephone sets are to be called, the calling signal generator 5 is required to have an output capacity of n/3 watt. However, in such a case where one third of the telephone sets are subjected to the ringing operation, only a load of n/9 is actually utilized, which leads to a problem of inefficiency.
Furthermore, when the ringing pattern is required to be changed to 1-second ON and 3-second OFF, it is necessary to modify the circuit configuration of the synchronization signal generator 51 so as to supply the extension control units 62 and 63 with synchronization signals having different periods. In addition, there also arises a problem that the contents of the correspondence table (Table 1) are required to be changed in the main memory of each of the extension controllers 62 and 63.